The present invention relates in general to charge pumps, and more particularly, to a charge pump comprising a feedback circuit which does not require a separate oscillator.
Many of today's integrated circuits (ICs) require multiple internal power supply voltages to perform specific functions. In one example, an RS232 communication IC employs both 5 volt and 10 volt supplies to translate the standard 5 volt signals to 10 volt signals prior to transmission. In another example, an MOS E.sup.2 PROM IC (Metal Oxide Semiconductor Electrically Erasable And Programmable Read Only Memory) uses a single 5 volt power supply for performing standard read operations; however, to execute a write cycle, the gate of a memory cell transistor requires a second power supply voltage, typically 20 volts. More common than not, as described in the preceding examples, the magnitude of the secondary supply voltage is greater than the primary supply voltage and, thus, the former is not readily obtainable from the latter. Furthermore, it is generally not cost effective to allocate an IC pin and apply an external supply voltage thereto just to satisfy an isolated circuit requirement. Hence, such ICs as described above typically incorporate a charge pump for internally generating the secondary voltage from the externally applied primary supply voltage to perform the specific functions.
A conventional charge pump typically comprises a fixed frequency oscillator operating open-loop and coupled to a buffer the latter of which sources and sinks current through a pump capacitor in response to high and low signals from the oscillator. First and second switching diodes are serially coupled between a voltage supply, V.sub.CC, and the output terminal. The pump capacitor is also coupled to the interconnection of first and second switching diodes. A load and filter capacitor are coupled to the output of the charge pump at the cathode of the second switching diode.
The energy transfer cycle of the conventional charge pump comprises two distinct phases; a charge phase and a discharge phase. During the charge phase the output signal of the oscillator is low causing the buffer to sink current from V.sub.CC through the first switching diode thereby storing energy in the pump capacitor. The second switching diode is reversed biased during the charge phase, therefore, the load consumes a portion of the energy stored in the filter capacitor decaying the output voltage. The buffer continues to sink current building the energy level of the pump capacitor to a maximum level until the output signal of the oscillator switches high commencing the discharge phase whereupon the buffer sources current through the pump capacitor and second switching diode thereby transferring energy from the pump capacitor to the filter capacitor and load. The first switching diode is reversed biased during the discharge phase isolating V.sub.CC from the load. The buffer continues to source current until the output signal of the oscillator again changes state completing the energy transfer cycle. Assuming that the load consumes less energy than the energy transfer capacity of the charge pump, the output voltage is theoretically maintained at a load dependent level approaching twice V.sub.CC minus the potentials across the first and second switching diodes.
In practice, the conventional charge pump is sensitive to the openloop timing mismatches between the period of the oscillator and the charging rate of the pump capacitor, the latter being dependent upon the current drive capacity of the buffer, the value of the pump capacitor and the often dynamic and difficult to predict loading conditions. If the time allocated to the charging cycle is too long, i.e., the frequency of the oscillator is too low, or the duty cycle too large, the pump capacitor charges to its maximum value followed by a dead period waiting for the oscillator to change state. The dead period may cause excessive discharge of the filter capacitor often resulting in unacceptable output voltage droops. For the case when the period of the oscillator is too short, the buffer does not have sufficient time to charge the pump capacitor to its peak value, thus, the average output voltage may decrease reducing the power available to the load. Ideally, the durations of the two states of the oscillator are equal to the times required for charging and discharging the pump capacitor.
Hence, there is a need for an improved charge pump with feedback such that the drive direction of the buffer is reversed upon sensing the completion of each charge and discharge phase substantially eliminating the timing mismatches of the frequency and duty cycle between the oscillator and pump capacitor and increasing the power available to the load.